1. Field of the Invention
This invention relates generally to devices for containing a liquid and maintaining the contained liquid such as coffee, at a fairly constant temperature, and to methods for maintaining liquids within a desired temperature range using such devices. Such devices include vacuum insulated flasks which maintain the temperature of the liquid stored within such flasks by imposing a thermal barrier which reduces the rate of heat transfer into or out of the container, and heater/flask systems, such as electric coffee makers, which, through the use of a resistance heater and either a glass or metal flask, maintain the temperature of the stored liquid by introducing heat to pre-heat the flask and subsequently, to maintain the temperature of the heated liquid in the flask.
2. Description of the Related Art
Although the device of the present invention is derived from and most closely related to the vacuum insulated flask, unlike ordinary vacuum insulated flasks, the device is adapted for use in hot beverage brewing and warming systems, such as those used to brew coffee or tea. In particular, the device relates to that portion of the art concerned with maintenance of the temperature of the brewed beverage, rather than heating of water and brewing the beverage. Most coffee and tea brewers on the market consist of a water heater of the continuous flow type into which cold water is introduced, a brewing chamber in which the coffee grounds or tea leaves are placed, a delivery system which introduces heated water into the brewing chamber, a storage flask in which the freshly brewed beverage is collected and a system for maintaining the temperature of the collected beverage within the storage flask. This invention relates to the system for maintaining the temperature of the collected beverage within the storage flask.
Ordinary silvered glass vacuum insulated flasks are not often used as collection devices in such systems, since, although they are able to slow the cooling of the beverage due to their insulating properties, these very insulating properties usually prevent heat from being introduced into the flask from an external heat source, thus lessening the desirability of using vacuum insulated flasks in such systems. Because of the inability to efficiently introduce heat into the vacuum insulated flask once the liquid is placed in it, the amount of time during which the liquid can be maintained at the desired temperature in vacuum insulated flask is a direct function of the flask's insulating properties and is thus limited.
In the present state of the art, the preferred storage flask in beverage brewing systems is made of single-walled, heat resistant glass. The temperature of the brewed beverage within the storage flask is maintained by an electric resistance heating element imbedded within or placed directly underneath a heat-conductive platform on which the storage flask rests. Higher-priced models have higher quality heating elements, thermostatic controls or more efficient conducting materials in the platform to create a more constant and even distribution of heat across the surface of the platform on which the storage flask rests.
Variations currently in existence include systems in which the heating element is integrated into the storage flask and systems in which an insulating jacket is wrapped around the storage flask. The combination of storage flask with heating element allows for a more efficient thermal coupling between the storage flask and the heating element. Often the integrated system also includes an insulated jacket surrounding the storage flask, to increase the unit's efficiency by reducing the amount of heat which must be introduced to maintain the brewed beverage at a desired elevated temperature.
The advantages obtained by integration of the storage flask and heating element are offset by several disadvantages. First, the integrated device is more difficult to clean. If the flask/heater is made sufficiently waterproof to allow for immersion cleaning, the cost of manufacture rises significantly. Finally, the cost of replacement parts is increased. The storage flask is the most often broken part of the system. Where the storage flask is integrated with a heating element, the cost of replacement is increased. Further, commercial coffee brewers often have replaceable heating platforms. Integration of the heating element into the storage flask makes replacement of broken or defective heating elements more difficult and costly. In general, such integrated systems tend to be preferred only when the need for portability and compactness outweigh these disadvantages.
As discussed above, prior to the present invention, use of double-walled vacuum insulated flasks in hot beverage brewing and storage systems was inefficient due to the difficulties of introducing heat into such flasks. However, some systems attempt to reduce the amount of heat required to maintain the temperature of the brewed beverage in the single-walled storage flask by surrounding the flask on its vertical surfaces with an insulating jacket. Use of an insulating jacket surrounding the exterior surface of such a storage flask provides some of the advantages of a vacuum insulated flask but with a significantly less efficient thermal barrier.
A disadvantage of temperature maintenance systems employing electric resistance elements, which to applicant's knowledge are employed in all hot beverage brewing/storage systems, in conjunction with single-walled storage flasks is that such heating systems tend significantly to accelerate the degradation of aroma and taste qualities of the stored hot beverage. In coffee, for example, many of the components of the taste and aroma of the beverage are heat labile. While a certain loss of taste and aroma over time is inevitable when the beverage is stored in an open container due to the volatility of certain taste components, this degradation of taste and aroma occurs much more rapidly in systems where temperature is maintained by introducing heat via electric resistance heating elements to compensate for the large heat losses in a non-insulated or poorly insulated, single-walled storage flask.
The increased rate of flavor deterioration is a function of the localized high temperature to which the stored beverage nearest the surface of the heating element is subjected and the amount of time the stored beverage is subjected to elevated temperature. In order to keep the liquid in the flask at a selected elevated temperature, the heating element must be at a significantly higher temperature than that desired for the heated beverage. The bottom (and/or side) of the flask which is in contact with the heating element is also at a higher temperature than that desired for the liquid. As the heat is conducted through the flask wall into the contained liquid, a thermal gradient is established, with the liquid directly adjacent to the heated flask wall being conductively heated to a higher temperature than that of the liquid as a whole. Although convection currents within the liquid dissipate the heat and limit the period of time when any particular portion of the liquid is exposed to the heated flask wall, such convection currents also assure that virtually all of the liquid has some exposure to temperatures significantly higher than the desired maintenance temperature of the hot beverage. This exposure of the liquid to the high temperature zone of the flask wall causes increased degradation of the heat labile elements of the beverage.
Units using insulated jackets on the storage flasks will maintain the heated beverage for a longer period of time without unacceptable degradation of flavor than will those without insulated jackets. The reasons for this increased performance is that the heating elements used with storage flasks which have insulated jackets can maintain the desired storage temperature while set at a lower temperature or by introducing heat less frequently. Because flavor degradation is a function of prolonged exposure to high temperature, the ability of these devices to use a lower temperature or to introduce heat less frequently reduces the rate of degradation. Unfortunately, as discussed above, units employing flasks with insulation jackets are more expensive to manufacture and more difficult to clean. Furthermore, cost of replacement flasks is higher.
Systems employing electric resistance conduction heating of a single-walled storage flask, including those with insulating jackets, having a further disadvantage in that the heating elements employed often heat unevenly with localized "hot spots". These localized areas create corresponding hot spots on the heating platforms and on the adjacent flask wall and expose the beverage contained in the flask to even greater temperatures than those required to maintain the temperature of the liquid. These hot spots result in more rapid deterioration of the taste and aroma elements of the stored beverage and occasionally introduce off-flavor elements by causing burns or scorching of the beverage.
One alternative to electrical resistance heating is microwave heating. Unfortunately, microwave heating also has significant disadvantages. The short wavelength microwave radiation generated by conventional microwave warming devices typically penetrates only about one-half inch into aqueous liquids. Thus microwave heaters also heat liquid unevenly, creating a "hot zone" near the surface of the liquid closest to the microwave source and depending on thermal currents in the liquid to distribute the heat throughout the liquid. Although microwave heating may not create the local zones of extremely high heat that electric resistance heat creates, it does heat liquids unevenly and the resulting hot zones can hasten the destruction of flavor and aroma elements in the heated beverage. In addition to the indirect, thermal effect mentioned above, microwave radiation also has a direct effect on many complex flavor molecules present in brewed beverages. Microwaves heat liquids by inducing translational and rotational vibrations in susceptible molecules and portions of molecules which absorb microwaves. The absorption of microwave radiation in itself can hasten the destruction of these molecules causing the flavor of the beverage to deteriorate. Microwave heating has a further disadvantage in that unshielded or poorly shielded microwave heat sources can be hazardous to the user. Microwaves have been linked to cataract development and can create interference with pacemakers. Thus, the design of a beverage warmer utilizing a microwave heat source requires that the adequate shielding be incorporated. The necessity of including adequate shielding as well as the relatively high cost of microwave heating elements are likely to cerate a cost disadvantage for devices containing microwave heat sources.
Accordingly, it would be a significant advance in the art to provide a means for storing hot beverages in a highly efficient insulating vacuum container, without the cost and convenience drawbacks associated with storage containers with insulating jackets, in combination with a means for introducing heat into the vacuum insulated flask and for preheating walls of the flask prior to receiving the hot brewed beverage. Such heat introduction means would desirably introduce heat into the contained hot beverage in a manner which avoids more than a transient exposure of the beverage to surfaces at elevated temperature, which avoids the risk of hot spots on flask surfaces in contact with the beverage, and which avoids hot zones and substantial thermal gradients in the beverage itself.
It therefore is an object of the present invention to provide a means for storing hot beverages in a vacuum insulated flask, means for preheating the vacuum insulated flask walls prior to receiving the hot beverage, and means for maintaining the temperature of the hot beverages by introducing additional thermal energy into the beverage in the flask by input of heat via a heat input portal in the vacuum insulated flask.
It is also an object of the invention to provide a method for forming heat input portals in double-walled vacuum flasks.
To my knowledge, the prior art is devoid of vacuum flasks utilizing heat input portals and heat sources to maintain the temperature of the contained liquid. A process is disclosed in Japanese Kokai Tokyo No. 81,46,553 issued Nov. 4, 1981 to Zojirushi Vacuum Bottle Co. Ltd. for creating a window in a vacuum bottle for the purpose of viewing the level of liquid inside. The process disclosed in this Japanese patent differs greatly from the processed disclosed herein. Further, the Japanese patent makes no mention of using the viewing window in any way for heating the contents of the vacuum flask.